Arrangement for compensation of ground offset in a data bus system

ABSTRACT

A description is given of an arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices ( 2, 10 ) which are each supplied with an operating voltage (U 0 ) by a voltage source ( 4; 14 ), are connected to ground (G 1 ); G 2 ) and have a data bus connection ( 6; 12 ) via which they are connected to a data bus line ( 8 ). The special thing about the invention is that between operating voltage (U 0 ) and ground (G 2 ) at least one voltage dividing device (R 3,  R 6 ) is connected whose output is coupled to the data bus connection ( 12 ) of at least one communication device ( 10 ) and whose voltage dividing ratio is selected such that an offset of the ground (G 2 ) of the communication device ( 10 ), whose data bus connection ( 12 ) is coupled to the voltage dividing device (R 3,  R 6 ), is in essence compensated compared to ground (G 1 ) of another communication device ( 2 ).

The invention relates to an arrangement for compensation of groundoffset in a data bus system comprising a plurality of communicationdevices which are each supplied with an operating voltage by a voltagesource, are connected to ground and have a data bus connection by whichthey are connected to a data bus.

Ground offset is understood to mean a potential difference betweendifferent ground points where the ground points may be assigned to thesame consumer of electricity or also to various consumers ofelectricity. There is a ground offset if the electrical connectionbetween the ground points has ohmic resistance. This is usually the casewith ground points that are relatively far apart, where there is anoticeable ohmic resistance because of the large distance. But also withrelatively small distances there may, under unfavorable conditions, be aground offset. Unfavorable conditions are usually found in vehicleswhere the ground points are present on the car body. A car body, whichis made of metal, is electrically conductive, it is true, but experiencehas taught that there are transitional resistors at many places whichresistors then cause a ground offset to occur.

A ground offset leads to a shift of potential which may considerablyaffect the functioning of the electrical system. Especially sensitiveare data bus systems when the voltage levels of the transmitted signalsare changed so much by the ground offset that they are no longerrecognized. Particularly in single-wire data bus systems a ground offsetleads to the breakdown of the communication relatively fast, because thereceiver no longer sees the voltage level necessary for reception on thedata bus line, for the case where the ground offset exceeds a certainmagnitude.

In DE 36 14 208 C2 is disclosed an arrangement for connecting a poweroutput circuit to a bus coupling device in which an intermediate circuithaving a Schmitt trigger behavior and a blocking input is inserted intothe data lines between the bus coupling device and the power outputcircuit and furthermore a high-ohmic barrier resistance is included inthat data line which runs from the bus coupling device to theintermediate circuit. As a result of the high-ohmic barrier resistance aquasi-separation of potential is achieved between the logic to the buscoupling and the output circuit comprising the power amplifiers. Thisprovides a large EMC stability of the whole circuit.

WO 95/34152 discloses a communication network transceiver which is usedin vehicles. The transmitter of the transceiver comprises an electronicground offset circuit which is connected to an independent DC groundreturn line and accordingly adjusts the voltage potential of the signalsto be transmitted in relation to the ground potential on this groundreturn line. Along with the data line this independent DC voltage groundreturn line leads to the receiver where there is a complementary groundoffset circuit in order to accordingly transform the voltage potentialof the received signals back then with regard to the ground ratioactually present there. This arrangement is suitable for two-wire bussystems, but not for single-wire bus systems.

GB 2 284 954 A shows a data bus driver generating a bias voltage, whichdriver in a central node of a communication network switches a biaslevel signal to each peripheral node via a data bus line in order toaccordingly cancel out different ground potentials.

U.S. Pat. No. 5,513,218 A describes a ground offset compensation circuitwhich comprises a what is called integrator which is used to simulatethe ground voltage of the line and apply this voltage to the ground ofthe transmitter. The integrator comprises a small current source and avery large current sink to be able to follow the ground potential of theline.

It is an object of the present invention to propose an arrangement forcompensation of ground offset which has a simple structure and isparticularly suitable also for single-wire data bus systems.

This object is achieved in that in the arrangement of the type mentionedin the opening paragraph at least one voltage dividing device whoseoutput is coupled to the data bus connection of at least onecommunication device and whose voltage dividing ratio is selected suchthat an offset of the ground of the communication device whose data busconnection is coupled to the voltage dividing device, is compensated inessence relative to the ground of another communication device.

The invention utilizes the recognition that because of a ground offsetthere flows an equalizing current over the data bus line between thecommunication devices. This equalizing current is reduced by thearrangement according to the invention of a voltage dividing device anda respectively selected voltage dividing ratio at least so much that theground offset is compensated in essence.

The invention is particularly advantageously suitable for single-wirebus systems, but is also recommended for two-wire bus systems with splittermination.

It is expedient that the voltage dividing ratio can be adjusted.

If the voltage dividing device comprises at least a first resistorconnected between the operating voltage and the data bus connection andat least a second resistor between the data bus connection and ground,the resistance value of the first and/or second resistor should bechangeable.

Alternatively, further resistors can be connected in series with thefirst and/or second resistor, which further resistors can optionally bebridged by further switching means.

In a further alternative embodiment further resistors which canoptionally be connected or disconnected can be connected in parallelwith the first and/or second resistor.

It is also conceivable for a current source to be connected in parallelto the first and/or second resistor.

It is expedient for an adjusting device to be provided for adjusting thevoltage dividing ratio. This adjusting device can accordingly controlthe resistance value of the first and/or second resistor or theswitching means for bridging parallel resistors or turning seriesconnected resistors on and off. The adjusting device may alsoaccordingly control the current source mentioned above.

Preferably, the voltage dividing ratio of the voltage dividing devicecan be set such that the no-load voltage on the bus connection of theassociated communication device corresponds to about half the operatingvoltage.

Particularly in order to limit the current, a terminating resistor maybe inserted between the data bus connection and the data bus line.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a circuit diagram of a single-wire bus system comprisingtwo communication terminals;

FIG. 2 shows the same circuit diagram as FIG. 1 with additionally asubstitute voltage source representing a ground offset;

FIG. 3 shows the same bus system as FIG. 1 in which the communicationterminals additionally have a simple voltage divider to increase theno-load voltage on their data bus connection; and

FIG. 4 shows a circuit diagram of part of a communication terminal withan extended voltage divider whose voltage dividing ratio can be adjustedin accordance with a first preferred embodiment of the invention and

FIG. 5 shows a circuit diagram of part of a communication terminalhaving a controllable current source for justifying the voltage dividingratio in accordance with the second preferred embodiment of theinvention.

FIG. 1 shows a typical single-wire bus system which comprises, forexample, two communication terminals. Single-wire bus systems arepre-eminently suitable in that the data bus line connecting thecommunication terminals has only one conductor and thus comprises only asingle “wire” and the individual communication terminals are furthermoreconnected to ground. Such a bus concept is used, for example, invehicles where the ground is formed by the body work which is made ofmetal.

The first communication terminal 2 shown in FIG. 1 comprises a powersupply or a voltage transformer 4 which is supplied with an externalvoltage U_(BAT) and generates internal operating voltage U₀ necessaryfor the modules of the communication node 2. Particularly in vehiclesthe external voltage U_(BAT) is customarily rendered available by abattery. The power supply or voltage transformer 4 respectively isfurthermore connected to ground G₁. Further is schematically shown asend transistor T₁ to whose collector is applied the operating voltageU₀ and whose base is connected to the connection T×D at which a transmitsignal is present. The emitter of the send transistor T₁ is switched toground G₁ via a first resistor R₁. Furthermore, the emitter of the sendtransistor T₁ is connected to a data bus connection 6 via a terminatingresistor R₂, to which terminal the transmit signal amplified by the sendtransistor T₁ is applied. The send transistor T₁ forms part of atransmitting stage not further shown here, in which the transmit signalmentioned earlier is generated. Furthermore is coupled to the data busconnection 6 a connection R×D₀ from which a receive signal is tappedwhich is applied to a receiving stage not shown in FIG. 1.

The first communication terminal 2 is connected to a data bus line 8 bya data bus connection 6, which data bus line 8 comprises a singleconductor or single “wire” respectively.

To this data bus line 8 is connected a second communication terminal 10via a corresponding data bus connection 12. The second communicationterminal 10 has a similar structure in the representation of FIG. 1 tothe first communication terminal 2. The second communication terminal 10also comprises a power supply or voltage converter 14 respectively whichis usually supplied with the same external voltage U_(BAT) as the firstcommunication terminal 2. The power supply or voltage transformer 14respectively is switched to ground G₂ and generates an internaloperating voltage U₀ which is applied to the collector of a sendtransistor T₂ whose base is coupled to a connection T×D to which atransmit signal is applied. The emitter of the send transistor T₂ isswitched to ground G₂ via a third resistor R₃. Furthermore, aterminating resistor R₄ connects the emitter of the send transistor T₂to a data bus connection 12. Also with the second communication terminal10 the send transistor T₂ in the representation of FIG. 1 forms part ofa transmitting stage (not further shown) in which the transmit signal isgenerated. Not only the transmit signal amplified by the send transistorT₂ is transmitted via the data bus connection 12, but also a receivingsignal is obtained which is fed to a receiving stage (not shown either)via a connection R×D₀.

If there is ground offset between the two communication terminals 2, 10,thus a potential difference between ground G₁ and ground G₂, this offsetcan be represented by a substitute voltage source V₁ as is schematicallyshown in FIG. 2. In a situation shown in FIG. 2 a voltage arises betweenthe first communication terminal 2 and ground G₁ as a result of theground offset. The result is an equalizing current I₀ flowing from thefirst communication terminal 2 over the data bus line 8 to the secondcommunication terminal 10, and a bias voltage being generated over theresistors R₃ and R₄, which voltage corrupts the input signal onconnection R×D₀. In this manner a voltage can be applied to the input ofthe receiving stage (not shown) which exceeds the receiving thresholdfor the recognition of a valid data signal, so that the communicationbetween the two communication terminals 2, 10 is disturbed or evencompletely prevented.

FIG. 3 shows an arrangement in which the no-load voltage has beenincreased to half the operating voltage U₀/2 by a simple voltagedivider. This is achieved by connecting a fifth resistor R₅ in the firstcommunication terminal 2 between the power supply or voltage transformer4 respectively and the terminating resistor R₂ and in the secondcommunication terminal 10 a sixth resistor R₆ between the power supplyor voltage transformer 14 respectively and the terminating resistor R₄.Therefore, in the first communication terminal 2 the resistors R₁ and R₅and in the second communication terminal 10 the resistors R₃ and R₆ forma simple voltage divider. The voltage to be tapped from the receivingstage (not shown) is now no longer found between R×D₀ and ground as inthe arrangement shown in FIG. 1, but between R×D₀ and another connectionR×D₁ which is coupled to the nodes between the resistors R₁ and R₅ or R₃and R₆ on the one hand and to the receiving stage (not shown in FIG. 3)on the other hand, and thus to the terminating resistor R₂ or R₄respectively. The problem of ground offset continues to be felt, it istrue, but now the additional voltage U_(BIAS) to be considered in asecond communication terminal 10 drops via the terminating resistor R₄.

FIG. 4 diagrammatically shows a simple concept for compensation ofground offset.

For this purpose, the communication terminal 10 whose ground G₂ in thearrangement shown lies at a lower potential compared to the ground G₁ ofthe first communication terminal 2 and is thus offset “downward”,comprises additional seventh, eighth and ninth resistors R₇, R₈ and R₉which are connected in parallel with the sixth resistor R₆ and can beswitched on by associated first, second and third switches S₁, S₂ andS₃. In this way a voltage divider that can be trimmed is formed in thesecond communication terminal, which voltage divider can be adjusted byselectively switching the additional resistors R₇, R₈ and R₉ on and off,so that the equalizing current I₀ and thus also U_(BIAS) caused by theground offset can be reduced or even completely eliminated. Switchingthe additional resistors R₇ to R₉ on is effected in dependence on thevoltage U₃ applied to the “downward” third resistor R₃. Usually, aminimum threshold is determined for this voltage U₃. The additionalresistors R₇ to R₉ are then to be connected such that the voltage U₃lies at least slightly above this threshold value. In this way a groundG₂ for the second communication terminal is raised to the same potentialas the “upwardly” offset ground G₁ of the first communication terminal 2and thus matched accordingly. At this point it should be observed forcompleteness' sake that instead of a three-stage implementation ofadditional resistors also a smaller or larger number of additionalresistors or even only a single additional resistor can be provided asdesired.

FIG. 4 further schematically shows the receiving stage as an operationalamplifier and featured by the reference numeral 16, whose one input iscoupled to R×D₀ and whose other input is coupled to R×D₁.

In the example of embodiment shown the voltage dividing ratio in thesecond communication terminal 10 is trimmed by selectively opening andclosing the switch S₁ to S₃. For this purpose, a control device 20 isprovided which measures for example via a measuring point 22 the voltageU₃ on the third resistor R₃ and, in dependence on a previously setthreshold, accordingly opens and closes the switches S₁ to S₃. This maybe advantageous particularly in case of fluctuating ground offset.

Other measures are conceivable too to set the voltage dividing ratio,and thus compensate the ground offset. For example, instead of theadditional resistors R₇ to R₉, a current source 24 may be inserted whichis accordingly controlled by the control device 20 as shown in FIG. 5.

Furthermore, it should be observed at this point that the possibility ofadjusting the voltage dividing ratio as is shown, for example, in FIGS.4 and 5, need not take place only in the second communication terminal10, but preferably also additionally in the first communication terminal2 in corresponding manner. For, as a rule, it is not known a prioriwhich of the communication terminals in operation has an “offset” groundwhich, for that matter, can also change during operation.

Finally it should be pointed out that the concept described hereinbeforecan also be used for compensation of ground offset for example fortwo-wire bus systems with what is called split termination.

1. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to the data bus connection of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, in which the voltage dividing device comprises at least a first resistor connected between the operating voltage and the data bus connection and at least a second resistor connected between the data bus connection and ground, characterized in that the resistance value of at least one of the first and second resistor can be changed.
 2. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to the data bus connection of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, and wherein the voltage dividing ratio can be adjusted; and the voltage dividing device comprises at least a first resistor connected between the operating voltage and the data bus connection and at least a second resistor connected between the data bus connection and ground, characterized in that further resistors are connected in series with at least one of the first and second resistor which further resistors can be bridged as required by switching circuits.
 3. An arrangement as claimed in claim 2, characterized by an adjusting device that controls the switching circuitry.
 4. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, and wherein the voltage dividing ratio can be adjusted; and the voltage dividing device comprises at least a first resistor connected between the operating voltage and the data bus connection and at least a second resistor connected between the data bus connection and ground, characterized in that further resistors are connected in parallel with at least one of the first and second resistor which further resistors can be switched on or off as required by the switching means.
 5. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to the data bus connection of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, in which the voltage dividing device comprises at least a second resistor connected between the data bus connection and ground, characterized in that a current source is connected in parallel with at least one of the first and second resistor and the voltage dividing ratio can be adjusted.
 6. An arrangement as claimed in claim 5, characterized by an adjusting device that controls the current source.
 7. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to the data bus connection of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, further including an adjusting device that controls the resistance value of at least one of the first and second resistor.
 8. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to the data bus connection of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, characterized in that the voltage dividing ratio of the voltage dividing device is set so that the no-load voltage on the data bus connection corresponds to about half the operating voltages.
 9. An arrangement for compensation of ground offset in a data bus system comprising a plurality of communication devices which are each supplied with an operating voltage by a voltage source are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by at least one voltage dividing device whose output is coupled to the data bus connection of at least one communication device and whose voltage dividing ratio is selected such that an offset of the ground of the communication device whose data bus connection is coupled to the voltage dividing device is compensated in essence relative to the ground of another communication device, characterized in that a terminating resistor is connected between the data bus connection and the data bus line.
 10. A data bus system comprising a plurality of communication devices which are each supplied with operating voltage by a voltage source, are connected to ground and have a data bus connection via which they are connected to a data bus line, characterized by an arrangement as claimed in any of the preceding claims. 